This invention relates to a humidity sensor made of metal oxide and adapted to detect a change in humidity as a change of an electrical resistance.
Generally, metal oxide has excellent water-absorbing and water-removing properties, and an electrical resistance element made of metal oxide has such a property that its resistive value varies due to a change in humidity. It is known to use the electrical resistance element as a humidity element by utilizing such property.
In a conventional humidity sensor made of metal oxide such as Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3, the powder of metal oxide is coated onto the surfaces of an inorganic insulating substrate to form a humidity sensing film. The sensor utilizes such a phenomenon that the electrical resistive value of the humidity sensing film varies due to a variation in humidity. The humidity sensor made of metal oxide has physically, chemically and thermally stable properties, but has a relatively high resistivity. Since a rate of change of a resistance resulting from a change in humidity is relatively small, it is difficult to detect a change of an electrical resistance with high accuracy. Thus, it is impossible to effect a humidity detection with high accuracy.
A humidity sensor is also known which utilizes the semiconducting property of an oxide having, for example, a spinel structure. The sensor has a relatively low resistive value and permits a detection to be made over a whole relative humidity range of 0 to 100%. However, if the sensor is left at room temperature, there is a tendency that its resistive value will be increased, failing to obtain a reproducible humidity sensor. In order to improve the reproducibility of the humidity sensor an attempt is made to heat the humidity sensor. However, the aging characteristic of electrode material of the sensor will be deteriorated by repetitive heating, resulting in a poor reliability and moreover, a complicated structure is necessary. Further, since a rate of change of a resistive value to a change of humidity becomes markedly greater, no better matching is obtained in connecting the sensor to a measuring circuit.